I am starting this thread, hoping that after everyone has chance to post, we define a clear methodology for capturing the output from the RF modulators of old computers and games from the early 1980s that were meant to use a TV set as a monitor.

I'll start with my own issue, the inability to capture the output of an old Sinclair ZX-81 computer. This thread also appplies to Timex-Sinclair, Atari, Commodore, Texas Instruments, Radio Shack, and other computers as well as numerous computer games that used TVs for the display.

Here is the issue:

1. The TV Channel output of an old Sinclair ZX-81 computer produces a picture on a TV screen when connected to the TV's 75 ohm RF inputs.

2. Connecting the TV Channel RF output of the Sinclair ZX-81 computer to the 75 ohm RF input of a capture card initially shows a few frames of the computer output, then the captured video fades to black.

3. Connecting the composite video out of a TV set that is showing the ZX-81 picture to the composite input of the capture card initially shows a few frames of the computer output, then the captured video fades to black.

4. Connecting that TV signal to the 75 ohm RF input of several VCRs and then connecting the composite video out of the VCR to the composite input of the capture card initially shows a few frames of the computer output, then the captured video fades to black.

I am hoping an expert out there can tell why the ZX-81 signal can be seen on TVs, yet the 75 ohm composite out of this TV image cannot be seen on a capture card except for a few frames.

Here are some questions:

1. Are capture cards pretty much the same these days in the way they accept and sync up with incoming signals, or are there capture cards that are known to work with the ZX-81 and other computer and games output? The WinFast card used in the tests appears to be using a ConExtant CX238880 chip or equivalent.

2. Are modern capture cards seeing the ZX-81 output as incomplete scan lines and therefore rejecting them? Note the narrow VirtualDub histogram screen shot of the ZX-81 output on the SuperGraphs web page.

3. Would some kind of external mixer or circuit that would blend a full scan black image with the ZX-81 composite signal allow a capture card to sync up with the mixed output and therefore allow capturing the ZX-81 output?

This is my first post here. I hope I have provided enough information for the discussion, If not, please let me know.

Connecting that TV signal to the 75 ohm RF input of several VCRs and then connecting the composite video out of the VCR to the composite input of the capture card initially shows a few frames of the computer output, then the captured video fades to black.

Have you tried first RECORDING the game action with your VCR - and then capturing from the tape?

1) Have you done this testing only WITH the switchbox included, or have you tried excluding the switchbox and directly connecting with what, if I remember correctly, is a composite connector (that connects to the switchbox at the top)?

2) (Unrelated to 1) above) Can't you have it set up "normally", so that the Sinclair's output is connected to your television and your television's composite output (If it has this) or RF output is connected to your VCR's composite input or RF output?

3) (Also unrelated)

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When the VCR RF pass through switch is set to TV, the ZX-81signal appears on the TV set, but when it is set to Video, the black image appears instead of the ZX-81 output.

I owned also a ZX-81 (24 years ago). As far as I remember, TV image was controlled by the Z80A microprocessor, not by an specialized chip. It's possible that syncro signals will be OK for a TV-set but unstable for other purposes. Try to get a time base corrector (TBC). Some high-end VCR has TBC and also exists as PCI cards.

1) Have you done this testing only WITH the switchbox included, or have you tried excluding the switchbox and directly connecting with what, if I remember correctly, is a composite connector (that connects to the switchbox at the top)?

I have not adapted the Sinclair RCA output directly to an "F" connector used for RF input these days, but I do not believe it would matter. The Sinclair uses an RCA cable, but it carries a 75 ohm RF signal, not a video composite signal. Like the Jensen transformer, the baluns in the switchbox isolate the signal and with the 75 to 300 ohm balun in the switchbox and the external 300 to 75 ohm balun at the F connector, I am back to a 75 ohm signal.

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2) (Unrelated to 1) above) Can't you have it set up "normally", so that the Sinclair's output is connected to your television and your television's composite output (If it has this) or RF output is connected to your VCR's composite input or RF output?

See my first post paragraph labeled #3. The composite output of the TV set only produces the ZX-81 output for a few frames, so it does not matter what equipment I connect the composite output of the TV set, there is nothing coming down the cable after a few frames.

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3) (Also unrelated)Did you set BOTH the television and the VCR to Ch.3?

Yes. In the last post I noted I had the VCR recording channel 3 from the cable TV cable while monitoring it on channel 3 on the TV set. I then unplugged the TV cable from the VCR RF input and replaced it with the ZX-81 output which was also channel 3. I confirmed that the VCR was getting the signal because when I switched the VCR RF pass-through direct to TV, the ZX-81 signal appeared on the TV. When I switched the VCR pass through back to VCR, the RF output to the TV went dark, the same RF output that showed TV channel 3 when I had the Cable TV cable attached to the VCR input.

I owned also a ZX-81 (24 years ago). As far as I remember, TV image was controlled by the Z80A microprocessor, not by an specialized chip. It's possible that syncro signals will be OK for a TV-set but unstable for other purposes. Try to get a time base corrector (TBC). Some high-end VCR has TBC and also exists as PCI cards.

I believe you are on to it. The screen shot of that VirtualDub histogram on the web page in my first post would indicate that the output signal is not like any normal NTSC signal.

I'm not very familiar with ZX81 (missed it just by little), but I have some thoughts/suggestions for you that might be useful:

- I don't quite understand all your connection schemes above, but from what I can see, the switchbox is an external device, right? Now, I suspect it does some "looping" because I see the 'fade to black' thing when a loop in the equipment is made, i.e. two VCRs have connected both inputs to outputs and vice versa or, on a modern computer, TVOut->VCR->Capture card of the same computer (and VCR tuned to see the output of the TVOut card).

- The successsor to ZX81 (a.k.a. ZX Spectrum or, I believe, Timex) had a built in RF modulator (wee, colour!), but also a composite signal could be pulled out of the edge-connector (just soldering 2 wires), and I (vaguely) remember recording it to VCR and/or displaying it on a pretty demanding/tight composite monitor, so I guess that signal was all right. But I highly doubt the same could be hacked on the ZX81. My guess would be that you would most likely need some kind of TBC.

I don't quite understand all your connection schemes above, but from what I can see, the switchbox is an external device, right? Now, I suspect it does some "looping" because I see the 'fade to black' thing when a loop in the equipment is made, i.e. two VCRs have connected both inputs to outputs and vice versa or, on a modern computer, TVOut->VCR->Capture card of the same computer (and VCR tuned to see the output of the TVOut card).

All those connections work with normal signals, but I was not going to even think about putting the ZX-81 anywhere near that maze. lol.

All the ZX-81 tests were done with equipment disconnected from the "maze" so the test would not be influenced by the loops you mention.

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The successsor to ZX81 (a.k.a. ZX Spectrum or, I believe, Timex) had a built in RF modulator (wee, colour!), but also a composite signal could be pulled out of the edge-connector (just soldering 2 wires), and I (vaguely) remember recording it to VCR and/or displaying it on a pretty demanding/tight composite monitor, so I guess that signal was all right. But I highly doubt the same could be hacked on the ZX81. My guess would be that you would most likely need some kind of TBC.

Brings back memories! Looks like the TBC is the only solution for the ZX-81's odd RF signal for any kind of direct usage. I wonder if anyone ever rents PCI cards? Somehow, they would have to do it for love not money.

If nothing else, I will set up a Canon Elura 100 Camcorder on a tripod, point it at the flat screen TV screen, record to a Mini-DV tape in the camera, firewire it into the computer to a file, and call it a day.

- Emulator(s). There must be one for ZX81. For spectrum, i believe a program existed that digitized audio casettes through sound card.

- (Very unlikely) Some years ago, PCI cards appeared that included whole 8-bit computers on them - but I guess they would be difficult to find/expensive...

- In light of AVIL's post, maybe the RF output of the ZX81 changes impedance under load (when connected to non-TV input) and overloads the poor Z80? Yeah, a longshot, but you seem experienced in electronics and maybe you can check...

Another possiblity (more affordable) are the cheap Macrovision fighters. I don't know any commercial name. His function is to reconstitute syncro signals altered by Macrovision system (anti-copy). Perhaps anyone can explain best this devices.

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maybe the RF output of the ZX81 changes impedance under load (when connected to non-TV input) and overloads the poor Z80

The Z80A don't drive directly video out. Instead, via output ports send signal to other chip (the Uncommited Logic Array). Z80A do the timing (in a poor way). The load is suffered by the RF modulator. If isn't damaged, it must cope with a standard device.

I'm surprised noone else has said this, but I'm guessing that your problem is that the ZX81 outputs a noninterlaced signal, which your capture device must not like. Do you have any other older 8- or 16-bit computers or game consoles to test it with? Most of them also output a noninterlaced signal. If this is a case, a TBC will definitely fix your problem, although you might do good to hack the Sinclair to gain access to the most pristine video signal the system offers, before RF modulating.

you might do good to hack the Sinclair to gain access to the most pristine video signal the system offers, before RF modulating.

Thanks, great suggestion. Here's the video output section of ZX-81 circuit, thanks to this amazing UK web site that is actually still selling the ZX-81 kits after all these years:

LocalH, does it look to you like like "16. TV/Tape" or "USA 3" would be good candidates for taking a pre-RF modulator signal? I am hoping the "USA 3" signal is close to an NTSC video composite signal.

Rather than run this directly into a composite input, I will probably buffer it with one of my video splitter circuits.

While I have a (c) on the simple circuit, anyone here is welcome to build the video splitting circuit for your personal purposes. I have had 3 splitters going continuously for 4 years in this setup.

They are great for losslessly sending a single composite output to multiple devices. Just build a separate circuit for each output needed from each source. I built three 2 way splitters so I could leave things connected all the time while using them in different ways. It did take some isolation transformers to keep the signals pristine because of some AC power source leg differences and distances.

Yeah, the input to USA 3 looks best. If I'm reading that schematic right, the video chip generates PAL-voltage signals no matter what, then the US system sends the signal through a resistor to drop it to NTSC level. But, as I said, some devices simply don't like noninterlaced signals, so using the composite may not give you any immediate benefits, but once you resolve that issue then you will get a cleaner image with composite. Too bad the system doesn't generate Y and C like the C64, or you could get an even cleaner signal.